1. Field of the Invention
The present invention relates to a sulfonium salt compound suited for use as a photo acid generator of a resist for lithography using far ultraviolet light, especially light of an ArF excimer laser, as exposure light, a photoresist composition, and a pattern forming method using the same.
2. Description of Prior Art
Higher density and higher integration of various semiconductor devices, which require a precision-precise working at a half-micron order, have been advanced conventionally. Therefore, requirements for the lithographic technology for formation of a precision circuit pattern have been more enhanced.
A means for refining a pattern is to use a lithographic light source having a shorter wavelength. Therefore, a KrF excimer laser having a shorter wavelength (wavelength=248 nm) is now employed in place of a conventional i-ray (wavelength=365 nm) in the mass production process of DRAM having an integration degree within a range of 256 Mbit to 1 Gbit (working size is within a range of 0.25-0.15 xcexcm). In the production of DRAM having an integration degree of 4 Gbit or more (working size is 0.15 xcexcm or less), which requires a more precisionprecise working technology, a light source having a shorter wavelength is required and it is considered to be effective to employ photolithography using an ArF excimer laser (193 nm) and a F2 excimer laser.
Particularly, ArF excimer laser lithography is considered to be as effective a precision working technology as KrF excimer laser lithography and is now studied intensively [see TAKECHI et al., Journal of Photopolymer Science and Technology, Vol. 5 (No. 3), pp. 439-446 (1992); R. D. Allen et al., Journal of Photopolymer Science and Technology, Vol. 8 (No. 4), pp. 623-636 (1995); and Vol. 9 (No. 3), pp. 465-474 (1996)].
Since the service life of a gas as a raw material of laser oscillation is short and expensive lenses are required and, furthermore, the lenses are liable to be damaged by light of laser, high sensitivity is required of a photoresist for the ArF excimer laser and F2 excimer laser described above, in addition to high definition which copes with the refinement of the working size.
As the photoresist having high sensitivity, a chemically amplified resist employing a photo acid generator as a photosensitive agent is well known. The chemically amplified resist is characterized in that protonic acid generated from the photo acid generator, which is a constituent component, as a result of light irradiation causes an acid catalytic reaction to a base resin, m which is a constituent component of the resist due to a heat treatment after exposure. Thus, the chemically amplified resist has attained rapid high sensitivity as compared with a conventional resist wherein the photoreaction efficiency (reaction per photon) is 1 or less.
Typical examples of the chemically amplified resist include a photoresist made of a combination of triphenylsulfoniumxe2x80xa2hexafluoroarsetate and poly(p-tert-butoxycarbonyloxy-xcex1-methylstyrene) described in Japanese Unexamined Patent Application, First Publication No. Hei 2-27660. It is now indispensable to develop a high sensitivity material which copes with the decrease of the wavelength of the exposure light source because almost all of photoresists to be developed are chemically amplified resists.
Examples of the photo acid generator include the triphenylsulfonium salt derivative described in Journal of the Organic Chemistry, Vol. 43, No. 15, pp. 3055-3058 (1978), the alkylsulfonium salt derivative such as cyclohexylmethyl (2-oxocyclohexyl) sulfonium trifluoromethanesulfonate disclosed in Japanese Patent Unexamined Application, First Publication No. Hei 7-28237, and the diphenyliodonium salt derivative and succinimide derivative described in Journal of the Polymer Science, Vol. 56, pp. 383-395 (1976).
In case the lithographic photoresist using a short-wavelength exposure light source, which is represented by the light of the ArF excimer laser, has low transparency, the definition is lowered and poor shape, such as trailing of the base of the pattern, is recognized. Therefore, an important technical problem of the photoresist for lithography includes, for example, improvement in transparency to exposure light.
Examples of the chemically amplified positive photoresist include those which contain at least three kinds of substances, for example, a photo acid generator, a base resin having an acid-decomposable group, and a solvent. The chemically amplified negative resist is classified into two types, that is, those which require a crosslinking agent and those which do not require a crosslinking agent. The chemically amplified negative resist, which requires the crosslinking agent, contains at least four kinds of substances, for example, a photo acid generator, a base resin capable of reacting with a crosslinking agent, a crosslinking agent, and a solvent. The chemically amplified negative resist, which does not require the crosslinking agent, contains at least three kinds of substances, for example, a photo acid generator, a base resin having a crosslinking group, and a solvent.
The photo acid generators, which are used most frequently in the ArF excimer lithography at present, are sulfonium salt compounds. Among these, a triphenylsulfonium salt derivative is used most frequently [see, for example, NOZAKI et al., Journal of Photopolymer Science and Technology, Vol. 10, No. 4, pp. 545-550 (1997); or YAMACHIKA et al., Journal of Photopolymer Science and Technology, Vol. 12, No. 4, pp. 553-560 (1999)].
However, the triphenylsulfonium salt derivative strongly absorbs far ultraviolet light having a wavelength of 220 nm or less, such as light of ArF excimer laser, because it has a benzene ring. Therefore, there was a problem that the transparency and definition of the photoresist are lowered when using the triphenylsulfonium salt derivative as the photo acid generator [see, for example, Takuya NAITO, the eighth lecture of the Society for the Study of Photoreaction and Electronic Materials, collection of abstracts of lecture, pp. 16-18 (1999)].
Therefore, xcex2-oxocyclohexylmethyl (2-norbornyl) sulfonium triflate and cyclohexylmethyl (2-oxocyclohexyl) sulfonium triflate, which are sulfonium salt derivatives having no photobenzene ring, were developed as the photo acid generator for ArF lithography [see Proceeding of SPIE, Vol. 2195, pp. 194-204 (1994); Proceeding of SPIE, Vol. 2438, pp. 433-444 (1995)].
However, these sulfonium salt compounds have problems such as poor heat stability. The heat decomposition point of these sulfonium salt compounds is within a range of 140-150xc2x0 C. in a state of a simple substance, and is lower in a resist film (resin film), e.g. 120-130xc2x0 C. Accordingly, the photoresist using such a sulfonium salt compound as the photo acid generator is likely to generate an acid even at the unexposed portion as a result of decomposition when heated to 130xc2x0 C. Therefore, there was a problem that the upper limit of the temperature in the heating process on formation of a resist film or the heating process after exposure is set to about 120xc2x0 C.
An object of the present invention is to obtain a photo acid generator which has high transparency to far ultraviolet light, especially light of ArF excimer laser, and also has excellent heat stability.
The present inventors have studied intensively and obtained a photo acid generator (sulfonium salt compound) which has high transparency to ArF excimer laser and also has high heat decomposition point of about 200xc2x0 C. Specifically, the present inventors have found that the above problems can be solved by a novel alkylsulfonium salt compound having a structure disclosed below, a photoresist composition containing the same as a constituent component, and a patterning method of patterning by light irradiation using the photoresist composition, thus completing the present invention.
The present invention provides a sulfonium salt compound represented by the following general formula (1): 
wherein R1 represents a butylene group, a pentylene group, a 2-oxobutylene group, or a 2-oxopentylene group, R2 represents a straight-chain, branched-chain, monocyclic, polycyclic or bridged cyclic 2-oxoalkyl group having 3 to 12 carbon atoms, or a straight-chain, branched-chain, monocyclic, polycyclic or bridged cyclic alkyl group having 3 to 12 carbon atoms, provided that either of R1 and R2 has an oxo group, and Yxe2x88x92 represents a counter ion.
In one preferred embodiment of the sulfonium salt compound of the present invention, the counter ion represented by Yxe2x88x92 in the general formula is Z-SO3xe2x88x92 (in which Z is CnF2n+1 (n is any one of 1 to 8), an alkyl group, or an alkyl-substituted or non-substituted aromatic group), BF4xe2x88x92, AsF6xe2x88x92, SbF6xe2x88x92, ClO4xe2x88x92, Brxe2x88x92, Clxe2x88x92, or Ixe2x88x92.
The sulfonium salt compound of the present invention has a high heat decomposition point and is superior in heat stability and transparency to light of ArF excimer laser. Therefore, it can be preferably used as a constituent component of a photoresist composition for lithography using far ultraviolet light, especially light of ArF excimer laser as exposure light.
The present invention also provides a photo acid generator comprising a sulfonium salt compound represented by the following general formula (1): 
wherein R1 represents an alkylene group, or an alkylene group having an oxo group, R2 represents a straight-chain, branched-chain, monocyclic, polycyclic or bridged cyclic alkyl group, or a straight-chain, branched-chain, monocyclic, polycyclic or bridged cyclic alkyl group, provided that either of R1 and R2 has an oxo group, and Yxe2x88x92 represents a counter ion.
In one preferred embodiment of the photo acid generator of the present invention, R1 represents an alkylene group having 4 to 7 carbon atoms, or an alkylene group having an oxo group, and R2 represents a straight-chain, branched-chain, monocyclic, polycyclic or bridged cyclic alkyl group having an oxo group, which has 3 to 12 carbon atoms, or a straight-chain, branched-chain, monocyclic, polycyclic or bridged cyclic alkyl group having 3 to 12 carbon atoms in the general formula (1).
In one preferred embodiment of the photo acid generator of the present invention, R1 represents an alkylene group having 4 to 7 carbon atoms, or a 2-oxyalkylene group having 4 to 7 carbon atoms, and R2 represents a straight-chain, branched-chain, monocyclic, polycyclic or bridged cyclic 2-oxoalkyl group having 3 to 12 carbon atoms, or a straight-chain, branched-chain, monocyclic, polycyclic or bridged cyclic alkyl group having 3 to 12 carbon atoms in the general formula (1).
In one preferred embodiment of the photo acid generator of the present invention, the counter ion represented by Yxe2x88x92 in the general formula (1) is Z-SO3xe2x88x92 [in which Z is CnF2n+1 (n is any one of 1 to 8), an alkyl group, or an alkyl-substituted or non-substituted aromatic group], BF4xe2x88x92, AsF6xe2x88x92, SbF6, ClO4xe2x88x92, Brxe2x88x92, Clxe2x88x92, or Ixe2x88x92.
The photo acid generator of the present invention is superior in transparency to light of ArF excimer laser and, therefore, it can be preferably used as a constituent component of a photoresist composition for lithography using far ultraviolet light, especially light of ArF excimer laser as exposure light. Since the photo acid generator of the present invention has high heat decomposition point and excellent heat stability, the upper limit of the temperature in the heating process on formation of a resist film or the heating process after exposure is not set to about 120xc2x0 C.
The present invention also provides a positive photoresist composition comprising the photo acid generator of the present invention.
The present invention also provides a negative photoresist composition comprising the photo acid generator of the present invention.
The photoresist composition of the present invention is suited for lithography using exposure light having a short wavelength, such as ArF excimer laser lithography, because of its high sensitivity and excellent definition.
Furthermore, the present invention provides a pattern forming method, which comprises forming a thin film on a substrate using the photoresist composition of the present invention, exposing to light having a wavelength of 300 nm or less, and developing to form a pattern.
In one preferred embodiment of the pattern forming method of the present invention, the exposure light is light of ArF excimer laser.
In one preferred embodiment of the pattern forming method of the present invention, the exposure light is light of F2 excimer laser.
Use of the pattern forming method of the present invention makes it possible to form a precision pattern required to produce a semiconductor device.